This invention relates to blind fasteners and more particularly to blind fasteners utilized in fastening sheets of material which are relatively soft.
As discussed in the co-pending patent application by John D. Pratt entitled "COMPOSITE BUCKLING BLIND FASTENER" Ser. No. 253,783 filed Apr. 13, 1981, now U.S. Pat. No. 4,457,652 and assigned to the same assignee as this patent application, three piece blind fasteners have been known and used for many years wherein the three pieces consist of a nut threaded onto a bolt and an expansible sleeve is located therebetween. In addition there are corresponding two piece blind fasteners wherein such sleeve is either missing or is initially integrally attached to the bolt head. A problem which has been encountered with respect to all such fasteners is that when completing the setting of such fastener substantial axial resistance is usually generated and, on occasion, can become so large as to shear off the bolt head thereby rendering the fastener useless. The main reason that such bolt head shearing occurs is that when the fastener is being set the setting action is caused primarily by the sudden contact of the nut nose with the bolt head which sets up sufficient axial resistance to cause the end of the bolt stem to sever at the breakoff neck. In other words, when the nut reaches the point of thread runout near the bolt head, an impulse load would be applied to the bolt head which may be so high in magnitude that breaks off the bolt head. It is the impulse of the contact between the nut and the bolt head which is designed to break the bolt stem at the breakoff neck but it may also break off the bolt head. Conventionally, such problem has been attacked by regulating the lubrication of the fastener parts so that sufficient torsional resistance is generated during the setting of the fastener and therefore the amount of axial resistance is thereby reduced. However, such approach requires careful control of the lubrication of such parts of the fastener parts and even then there are problems of reproducibility.
Such problem has been substantially aggravated by the advent of the useage of relatively soft material such as a graphite composite or the utilization in structures in which very thin sheets of material are being utilized. In such situation the force applied by the sleeve or bolt head on the blind side surface may be sufficiently high to deform the blind side surface and weaken the sheet at that point.
Consequently, an object of the present invention is a blind fastener which can substantially reduce the impulse on the bolt head during the completion of the setting of the fastener.
Another object of the present invention is a blind fastener which substantially increases the torsional resistance during the completion of the setting of the fastener compared to the axial resistance.
Still another object of the present invention is a blind fastener which has improved locking action.
Another object of the present invention is a blind fastener having an altered thread section in the bolt stem adapted to compensate for the varying tolerances of the fastener parts and to permit substantial clamping action during the completion of the setting of the fastener without shearing off the bolt head.
Other objects and advantages in the present invention will be readily apparent from the following description and drawings which illustrate a preferred exemplary embodiment of the present invention.